changing profile in bbpress blanks the wordpress 'display name' field

in wordpress’ admin screens, many author names are displayed in brackets or are entirely blank.

in the user profile area of the wordpress admin interface, these users appear to have a display name – but when checking phpmyadmin, i realise they actually have a blank display_name field in the wp_users table.

if i save their profile in wordpress (without touching anything), the display name is set again in the database and all is fine. if i then save their profile in bbpress, their display name disappears again.

this suggests that saving profiles via the bbpress profile page blanks out the display_name field.